Thermal phase and excitonic connectivity in fluorescence induction

Chl fluorescence induction (FI) was recorded in sunflower leaves pre-adapted to darkness or low preferentially PSI light, or inhibited by DCMU. For analysis the FI curves were plotted against the cumulative number of excitations quenched by PSII, n (q), calculated as the cumulative complementary area above the FI curve. In the +DCMU leaves n (q) was < 1 per PSII, suggesting pre-reduction of Q (A) during the dark pre-exposure. A strongly sigmoidal FI curve was constructed by complementing (shifting) the recorded FI curves to n (q) = 1 excitation per PSII. The full FI curve in +DCMU leaves was well fitted by a model assuming PSII antennae are excitonically connected in domains of four PSII. This result, obtained by gradually reducing Q (A) in PSII with pre-blocked Q (B) (by DCMU or PQH(2)), differs from that obtained by gradually blocking the Q (B) site (by increasing DCMU or PQH(2) level) in leaves during (quasi)steady-state e(-) transport (Oja and Laisk, Photosynth Res 114, 15-28, 2012). Explanations are discussed. Donor side quenching was characterized by comparison of the total n (q) in one and the same dark-adapted leaf, which apparently increased with increasing PFD during FI. An explanation for the donor side quenching is proposed, based on electron transfer from excited P680* to oxidized tyrosine Z (TyrZ(ox)). At high PFDs the donor side quenching at the J inflection of FI is due mainly to photochemical quenching by TyrZ(ox). This quenching remains active for subsequent photons while TyrZ remains oxidized, following charge transfer to Q (A). During further induction this quenching disappears as soon as PQ and Q (A) become reduced, charge separation becomes impossible and TyrZ is reduced by the water oxidizing complex.